This invention is concerned with an apparatus for laser welding of metals in a pressurized atmosphere.
Oxide Dispersion Strengthened (ODS) alloys, such as those described in U.S. Pat. No. 3,992,161, are generally produced by powder metallurgy techniques. The powdered metals used for mechanically alloyed ODS alloys, as well as a number of other alloys, can be prepared by atomizing a molten metal stream with high velocity jets of argon gas. Some of the argon used for atomization, as well as argon which may be incorporated during a mechanical alloying operation performed in an argon atmosphere, can be entrapped in the powdered metal and retained, albeit in small quantities, in consolidated and sintered shapes prepared from the ODS alloys. Attempts to join ODS alloys by welding have been hampered by gross porosity resulting from entrapped argon.
An additional welding problem resides in the agglomeration and slagging-off of the dispersoid without which the ODS alloy weld deposits are severely limited in elevated temperature strength. Attempts to weld the ODS alloys with a variety of well known fusion welding techniques have generally been unsuccessful in overcoming the dispersoid agglomeration problem.
The advent of LASER (Light Amplification by Stimulated Emission of Radiation) devices useful for welding provides an advantagous tool for attacking the problems inherent in joining ODS alloys. Because a laser beam provides an intense source of radiation in a small area, rapid melting occurs and under appropriate conditions can be followed by rapid solidification so that the dispersoid will have insufficient time to agglomerate and/or slagg off (e.g., melting and solidification during laser welding can occur in 8 milliseconds). Thus, under appropriate welding conditions, it is theoretically possible for the dispersoid to be retained within the weld deposit and to provide the desired strengthening effect during elevated temperature service. However, the problem of argon induced porosity is not avoided by using known laser welding techniques, similar to those described in U.S. Pat. Nos. 3,824,368 and 4,000,392, where inert gas substantially at atmospheric pressure and subject to ingestion of air is used to shield the workpiece.
Porosity, for example, of the type attributed to the presence of argon in ODS alloys, can be largely suppressed by welding in a pressurized atmosphere containing gas of essentially the same composition as that responsible for the porosity, e.g., argon. However, attempts to use a laser beam for welding in a pressurized chamber containing such an argon atmosphere have been thwarted by ionization of the still argon and the formation of a soot-like buildup in the welding chamber which absorb laser power and interrupt welding continuity. In addition, a stream of what is believed to be ionized metal particles travels along the laser beam, impinging on the inside surface of the light transmitting means (sight glass necessary for transmission of the laser beam into the chamber). This action causes absorption of laser energy as well as severe thermal stress and rapid breakage or even melting of the light transmitting means.
The latter problem appears, in many respects, to be unique to laser welding within a chamber, although laserinduced debris problems have been described in several patents dealing with laser cutting and drilling as represented by U.S. Pat. Nos. 3,626,141, 3,866,398 and 4,027,137.